Process for atomising electrolytes and the chemical analysis thereof

ABSTRACT

The present invention relates to a one-step process useful in the chemical analysis for atomising electrolyte solutions or fluids with electric conductivity, and a multielement direct analysis method of the components of such fluids, which is also operable in a monitoring mode. In this analysis method, the one-step atomisation of the fluid is followed by optical and/or mass spectroscopy determination of the sample components. The atomisation process is characterised by generating an electric discharge of 1000 to 5000 V/cm on the surface of said electrolyte between the fluid as cathode and the anode positioned in the gas atmosphere above the said fluid.

TECHNICAL FIELD

The present invention relates to a one-step process useful in thechemical analysis for atomising electrolyte solutions or fluids withelectric conductivity, and a multielement direct analysis method of thecomponents of such fluids, which is also operable in a monitoring mode.In this analysis method, the one-step atomisation of the fluid isfollowed by optical and/or mass spectroscopy determination of the samplecomponents.

BACKGROUND ART

In many fields of optical spectroscopy (atomic absorption or atomicemission spectrophotometry, atomic fluorescence) and mass spectroscopythe precondition of measurement is the existence of the components in anatomic vapour state. The term: "atomisation" defines a complex processwherein an atomic aerosol is obtained from the sample by one ormultistep energy transfer. In the following, the term: "atomisation" isused in this sense.

According to the prior art, most of the atomisation processes used foranalytical purposes prepares first a coarse aerosol by pneumaticvaporisation of the electrolyte solution, which is then transferred intoproperly selected chemical flame of high temperature or plasma flamegenerated by high frequency (Boumans, editor: Theory of Atomic EmissionSpectroscopy). In these processes the vaporisation step, due to thesmall sample requirement, is performed by capillary blotting orblow-slot method and accordingly, they can be used in the analysis ofsolutions which are predominantly clear and/or contain little amount offloating material, which would mean the necessity of inserting apre-filtering step before vaporisation in many practical processes. Themass flow of atomisation, i.e. the amount of electrolyte atomised in atime unit is determined by the atomisation velocity, thus, regularcleaning of the vaporisation system and permanent control of the flowand pressure conditions is necessary. Another drawback of such processesis that atomisation by chemical flames needs a permanent and strictcontrol due to the extraordinarily flammable and explosive gases(acetylene, dinitrogen oxide, oxygen) used therein. This disadvantagedoes not appear in the high frequency-generated plasma flameatomisation, but the high frequency unit is of complicated structure andhigh energy consumption.

Another method for solution atomisation used for analytical purposes isthe electro-thermal, heating of a precisely determined quantity ofelectrolyte by programmed steps of evaporation, drying and vaporisation,the so-called graphite furnace atomisation. This method provides anadvantageous high atom vapour concentration, however, the process iscomplicated and consists of multiple steps, automation of sampleinjection is difficult to accomplish, and continuous working mode isimpossible due to permanent contamination of the graphite furnace by thenon-evaporating parts of the electrolytes.

In the optical spectroscopy, other methods for quasi-continuousatomisation and analysis connected therewith in dude "weeping carbonelectrode" method (Feldman, C.: Anal. Chem. 21: 1041/1949/), "capillaryelectrode carbon arc" method (Nalimov, V. V.: Zav. Lab. 23: 1351/1955/,Zink, T. H. Appl. Spectr. 13: 94/1959/), rotating disc electrode sparkexcitation (Pierucci, M. et al: Nuovo Cimento 17: 275/1931/) and feedingelectrolyte in the spark gap (Twyman, F. et al: Proc. Roy. Soc. 133:72/1931/). These methods have never been widely used due to their commondisadvantage that during atomisation, irreversible deposits are formedon the solid electrodes, and accordingly, they cannot be used incontinuous technologies.

As summarized above, the known analytical methods based on analyticalchemical atomisation processes of electrolyte-containing fluids (atomicabsorption, atomic emission, atomic fluorescence, mass spectroscopy)require a very thorough sample preparation, (filtration, recovery,dissolving), precise solution administration, permanent control(explosion risk in technology) and they are not suitable for continuous,control-free (monitoring) system.

Accordingly, the present application aims to provide a one-step methodfor atomising electrolyte solutions and an atomic spectroscopy basedthereon for a multielement analysis, which eliminate the abovedisadvantages.

DISCLOSURE OF THE INVENTION

The invention is based on the recognition, that atomisation of anelectrolyte may be carried out by generating an electric gas dischargeof 1000 to 5000 V/cm intensity on the surface of the electrolyte betweenthe fluid as cathode and an anode arranged in the atmosphere above theelectrolyte. It has been found that as an embodiment, the followingmethod is suitable for one-step direct multielement analysis:

The pH-value of the fluids is optionally adjusted to acidic, preferablyin the interval of 1,0 to 2,0, a part of the fluid is atomised bygenerating electric gas discharge on the surface of fluid between theelectrolyte as cathode and the anode arranged in the gas atmosphereabove the electrolyte, and the constitution of the sample is determinedby methods known per se, by optical spectrometry based on light emissioncreated by excitation of the atomic vapours obtained from thecomponents, or by direct mass spectrometric analysis of the atomicvapours.

It has also been found that the presence of an extremely high amount ofalkali earth metal in the sample (10 to 100 g/l) can substitute theacidifying step, and by using the above electrode arrangement and fieldstrength interval, both the atomisation and the analysis connectedtherewith can be carried out.

According to a preferred embodiment of the invention the atomisation ofthe electrolytes can be carried out by generating a direct current gasdischarge of a field strength from 1000 to 5000 V/cm on the fluidsurface between the electrolyte as cathode and the anode positioned inthe vapour atmosphere above the electrolyte. The field strength can becontrolled by the voltage and the distance between the electrode. Theatomisation mass flow, i.e. the quantity of the sample atomised andentering the plasma in a time unit reaches a defined, constant value byadjusting the field strength and the current rate to a constant value,and thus, sample administration becomes superfluous.

In the gas discharge process, only atomisation of the cathode takesplace; the anode serves for conducting away the electrons formed in thedischarging process (G. Francis: Glow Discharges at Low Pressures,Handbuch der Physik Vol. XXII, Springer V. Berlin 1956). Thus thematerial of anode does not disturb the process provided that it issufficiently heat-resistant.

SUMMARY OF THE DRAWINGS

FIG. 1 shows a possible embodiment of the atomisation.

FIG. 2 shows a preferred embodiment of the pulse generation and timeresolution measurement

BEST MODE OF CARRYING OUT THE INVENTION

A possible embodiment of the atomisation is shown in FIG. 1. In thedischarge cell made of a non-conducting material (glass) (1) aheat-resistant anode electrode (Tungsten) (2) is inserted to a distanceof 3 to 6 mm above the surface of the electrolyte cathode (5). Theelectrolyte cathode (5) is in electric contact with the electrode (3)via the diaphragm (4). The electrode (3) is placed in the pole space ofthe effluent electrolyte (9). The electrode (3) is connected with thenegative pole of a high voltage power supply, while the electrode (2) isconnected to the positive pole thereof. The light emission of thedischarge plasma (7) forming in the gas region (6) is detected by anoptical spectrometer (12) directed in the observation direction (10).The sample is fed into the cathode space. (5) through the connection(8).

The atomisation can be carried out by single sample injection or duringflow of the fluid containing the electrolyte. Flow of the solutioncontaining the electrolyte in the gas discharge pole space is especiallypreferred as the flow of fluid can also be used for heat transfer. Inthe single sample injection mode, cooling can be accomplished e.g. in acell with coolant jacket.

The preparatory process of the electrolyte solutions for analysisoccasionally needs a pretreatment step. If the sample is a dilutesolution, pretreatment constitutes adjustment of the pH value, which isaccomplished suitably with mineral acids, preferably to a pH value of1,0 to 2,0. Control of the adjustment in a static or dynamic system maybe carried out in methods known per se, e.g. by pH measurement.

Should the sample contain the alkali and alkali earth metal salts in ahigher concentration, e.g. 10 to 100 g/l, acidification may be omitted.

In one embodiment, wherein the electrolyte solution contains less than 2g/l alkali metal and alkali earth metal ions, the pH of the electrolytesolution is adjusted to a value of 0.5 to 3, preferably to a value of 1to 2, by the addition of an acid. In one embodiment, wherein theelectrolyte fluid contains from 2 to 10 g/l alkali metal and alkaliearth metal ions, the pH of the electrolyte solution is adjusted to avalue of 3 to 5 by the addition of an acid.

The gas plasma atomised according to the invention excites a part of thecomponents of the atomic vapour to light emission. This light radiationforms the basis of the optical spectroscopy analysis. Accordingly, theintensity of light radiation emitted by the excited components ofelectrolyte solutions induced by one-step atomisation, such as metalatoms, can be determined by direct optical spectroscopy methods, whichprovides signals having intensity proportional with the concentration ofa component at a wave length characteristic to each component. Thenon-excited components can be analysed by directing them out of theatomised gas atmosphere, e.g. by vacuum, and by mass spectroscopymethods known per se.

The interfering background radiations known from optical emissionspectroscopy (e.g. the bands of radicals formed from the solvent and thecomponents of the gas atmosphere, resp.), are also present in theemission spectrum of the plasma atomised according to the invention,and, in certain ranges, they substantially reduce the accuracy of theanalytical measurement by increasing the background noise.

It has also be found that the band radiation of the atomized gas plasmamolecules and radicals according to the invention can substantially bereduced periodically by using additive current impulse. The direction ofthe applicable current impulse is identical with that of the dischargesupporting current, its strength is 2 to 10 times higher, the term is100 to 1000 microseconds, the upper limit of the duty cycle is about20%, i.e. 1 to 4 pulse:interval ratio.

The intensity of the light emitted should suitably be determined in thesecond half of the current pulse, or in the term 10 to 100 microsecondsafter the end of the current pulse, wherein the intensity of thebackground signal decreased to a low level, or, in the latter caseapproached or reached the dark current level of the detector.

A preferred embodiment of the pulse generation and time resolvedmeasurement is shown in FIG. 2. A section of the field current increasedduring one pulse is illustrated in FIG. 2a. On the basic dischargecurrent (1) a current pulse (2) of 100 to 1000 microseconds issuperimposed. FIG. 2b shows the concurrently registrable detector signalfunction. Curve 3 corresponds to the intensity of the band backgroundradiation, curve 5 is a typical example of the atomic line intensityduring the current pulse and closely before and after thereof. The lineintensity (5) measured during the interval (6), was improved to 2 to5-fold of the signal/background ratio (compared to background 3) and 2to 10-fold of the pulseless direct current excitation. The term (7) is abackground decay period after the current impulse (2), wherein theintensity of plasma emission might decrease almost to zero.

The pulse excitation and time resolved measurement method according tothe invention can be evaluated with a calibration curve taken betweenthe signal intensity during the time intervals 6 or 7 on a wavelengthcharacteristic to the target component and the concentration of saidcomponent.

The possibility of continuous sample feeding constitutes one of thegreatest advantages of the present invention, as the method can be usedin complex industrial and environmental monitoring systems. A furthergreat advantage of the multielement analysis according to the inventionis that the known processes are substantially simplified by eliminatingthe preparation of samples through controlled mass flow directatomisation, i.e., without pulverisation, drying and thermalvaporisation.

In the process according to the invention the atomisation mass flow is 1to 2 orders of magnitude smaller than that of known atomisationprocesses, and accordingly, the sensitivity of the method is at leastone order of magnitude less than that of the laboratory atomicspectroscopy methods. However, this sensitivity of the analytical methodaccording to the invention is suitable in most of the industrial,environmental, etc., analytical processes.

The progressive effects of the invention can be summarised as follows:

one-step, direct atomisation by atomisation mass flow controlled by thefield strength and discharge current strength,

the atomisation takes place on the free fluid surface of the sampleflow, a permanently recovering surface is formed, i.e. the samplepreparatory line needs not more than an eventual crude filtration,

a steady state is formed by permanent ventilation of the recoveringfluid surface and the gas space, as well as the heat removal, i.e. thereis no deposition surface, and atomisation process can be maintainedcontinuously as well.

We claim:
 1. A method for atomizing an electrolyte fluid, said methodcomprising the step of generating a gas discharge by providing a fieldstrength of 1000 to 5000 V/cm between a surface of said electrolytefluid, as cathode, and an electrode, as anode, positioned in anatmosphere above said surface.
 2. The method of claim 1, wherein saidelectrode, as anode, is a heat resistant material.
 3. The method ofclaim 1, wherein said electrode, as anode, is positioned 3 to 6 mm abovesaid surface.
 4. The method of claim 1, wherein said electrolyte fluid,as cathode, is an electrolyte solution.
 5. A method for elementalanalysis of electrolyte fluid, said method comprising the steps of:(a)generating a gas discharge by providing a field strength of 1000 to 5000V/cm between a surface of said electrolyte fluid, as cathode, and anelectrode, as anode, positioned in an atmosphere above said surface; and(b) determining the elemental constitution of said gas discharge.
 6. Themethod of claim 5, wherein said elemental constitution of said gasdischarge is determined by optical spectrometry.
 7. The method of claim5, wherein said elemental constitution of said gas discharge isdetermined by mass spectrometry.
 8. The method of claim 5, wherein saidelectrode, as anode, is a heat resistant material.
 9. The method ofclaim 5, wherein said electrode, as anode, is positioned 3 to 6 mm abovesaid surface.
 10. The method of claim 5, wherein said electrolyte fluid,as cathode, is an electrolyte solution.
 11. The method of claim 5,wherein said electrolyte fluid contains less than 2 g/l alkali metal andalkali earth metal ions, and the pH of said electrolyte fluid isadjusted to be from 0.5 to
 3. 12. The method of claim 5, wherein saidelectrolyte fluid contains less than 2 g/l alkali metal and alkali earthmetal ions, and the pH of said electrolyte fluid is adjusted to be from1 to
 2. 13. The method of claim 11, wherein the pH of said electrolytefluid is adjusted by the addition of a mineral acid.
 14. The method ofclaim 5, wherein said electrolyte fluid contains from 2 to 10 g/l alkalimetal and alkali earth metal ions, and the pH of said electrolyte fluidis adjusted to be from 3 to
 5. 15. The method of claim 14, wherein thepH of said electrolyte fluid is adjusted by the addition of a mineralacid.
 16. The method of claim 5, wherein said field strength is providedby applying a current comprised of current pulses superimposed on a basecurrent of the same sign.
 17. The method of claim 16, wherein saidcurrent pulses have an intensity 2 to 10 times greater than theintensity of said base current, a term of 100 to 1000 microseconds and amaximum duty cycle of 20%.
 18. The method of claim 17, wherein theelemental constitution of said gas discharge is determined during thelatter half of said current pulse term.
 19. The method of claim 17,wherein the elemental constitution of said gas discharge is determined10 to 100 microseconds after said current pulse term.